I have started to setup my own joomla webserver using Ubuntu 9.10 server and Joomla 1.5.15.
After the installation everything works fine using http. But before going online I wanted to secure the adminstrator part using ssl (https) in combination with a client certificate. I've configured ssl in apache (httpd.conf) and added the following lines to activate the security for the administrator:

Thank you for the swift response. I was probably not clear enough. In actuallly did install the certificates following the procedures described by vanemery. The admin website works fine under https. I've also protected myPhpAdmin with ssl and this also works fine. The only thing that doesn't work is uploading files. Uploading packages using the extension manager as well as uploading images using the media manager gives the same error. Is there a restriction that prevents file upload using https?

Uploading files less than this max size worked fine.
Browsing around in the apache community I found the cause is a SSL handshake buffer that is intentionally limited in size to prohibit deny of service attacks. This is solved in apache 2.2.12 up by introducing the SSLRenegBufferSize directive. Fortunately I am using 2.2.14 so adding this directive with a 10M limit solved the issue: